1. Field of the Invention
The present invention generally relates to an apparatus for detecting a level of liquid which is stored in a reservoir. More particularly, the present invention pertains to an apparatus used in an engine, for detecting the level of engine oil, and for warning the driver when there is a shortage of the engine oil.
2. Description of the Related Art
In general, a level warning system for engine oil is known, which includes a level sensor for detecting the level of the engine oil. For example, a "TOYOTA CROWN MAJESTA NEW CAR MANUAL" (published on October, 1991) discloses a conventional and typical oil level warning system.
According to this publication, an oil level sensor is attached to an oil pan which is secured to the bottom portion of an engine. The oil level sensor includes a reed switch and a thermal switch. A float, which is floatable on the oil, is provided around the reed switch to be vertically movable. The reed switch changes its condition between the "ON" and "OFF" conditions, in accordance with the position of the float (or float height). The thermal switch changes its condition between the "ON" and "OFF" conditions, in accordance with the engine oil temperature.
The warning system includes an electronic control unit circuit (ECU circuit) which is exclusively used for warning, and a display unit mounted in an instrument panel at a driver's seat. When the ECU circuit receives a signal transmitted from the oil level sensor, which indicates that the oil level is below the requirement level, the ECU circuit transmits a signal to the display unit, so as to display a warning sentence to notify a driver that the engine oil is in the insufficient condition.
When the engine oil temperature is less than a certain temperature, the viscosity of oil is relatively high. In this case, the volume of oil supplied from the oil pan to every part of the engine by means of an oil pump exceeds the required volume. In other words, the volume of oil left in the oil pan is relatively small. This kind of condition or phenomenon is called "Oil Rise". The Oil Rise may cause the oil level to be judged as decreased, even when a sufficient volume of oil is originally supplied.
Considering this condition, according to the conventional warning system, when the oil temperature is low, the thermal switch is in the "ON" condition. When the thermal switch is in the "ON" condition, the ECU circuit never permits the display unit to show the warning sentence which indicates the engine oil shortage, regardless of the reed switch condition. On the other hand, when the oil temperature is above a predetermined temperature and the oil level is below a predetermined level, both the thermal switch and the reed switch are in the "OFF" condition. At this time, the ECU circuit permits the display unit to show the warning sentence that indicates the oil shortage.
For example, when a vehicle is cornering, the oil level sensor may fail to detect a precise oil level, because of the sway or surging of the oil surface. To avoid the mal-detection, the conventional warning system further includes a delay timing circuit which delays the operation for detecting the switching of the signal transmitted from the reed switch by a predetermined period of time (approximately 40 seconds). Even when the reed switch temporarily detects a low oil level within the predetermined delay period of time, the ECU circuit ignores this temporal detection, by the action of the delay timing circuit. Therefore, the display unit never shows the warning sentence which indicates the oil shortage, based on the temporal detection of the low oil level.
However, requiring the ECU circuit and installing the delay timing circuit thereinto causes the entire circuit configuration of the system to be complicated and enlarged. As a result, the manufacturing cost of the system is increased.